My Dome Cake Doomed!

I made a dome cake recently, at first the shape held up but the very next day it dropped an inch and lost its dome shape. I stored it in the fridge overnight but it still dropped. The cake was made up of chiffon and I filled it with chocolate mousse then I used ganache to cover the cake. Can anybody tell me what went wrong here? Do you think I should have placed it in the freezer instead? Would the mousse be still ok if I had? Please help.

hi! i carved the cake and pressed it at the bottom of the pan then i put in the mousse then covered it with another cake layer as bottom. i stored it that way in the fridge for a few hours then i inverted it and poured ganache. then i placed it back in the fridge so the ganache would stiffen. i checked it the very next day and to my surprise it deflated. im pretty sure i've put in enough mousse to fill up the cake and im also pretty sure my cake has already cooled before i assembled everything. and the ganache is not even hot when i poured it onto the cake. this is really a puzzle to me.